According to the current practice, copper is recovered by leaching copper bearing ores with weak sulfuric acid solution. Various leaching methods are described by Allison Butts in "Copper-- The Science and Technology of the Metal, Its Alloys and Compounds", Hafner Publishing Company, Inc. 1970, pages 307 through 314. In addition to leaching methods described by A. Butts, recent technology allows recovery of copper from ore deposits by in-situ leaching. It is to be understood that the invention described herein has general applicability to the recovery of copper from any dilute acidic copper-bearing solution from any source. For the purposes of this disclosure, however, pregnant leach solutions will be described as having been obtained from heap or dump leaching and in-situ leaching.
A pregnant leach liquor from any of the aforementioned leaching operations may contain from about 0.1 to 10 grams or more per liter of copper. A significant concentration of iron as both ferrous and ferric iron may also be present in some of the pregnant leach liquors. Heretofore, it has been a standard practice to recover copper by contacting the acidic leach solution with iron such as in launders or cones whereby copper is precipitated from the solution. Because of the composition of the leach liquor, several chemical reactions occur in the cementation step, i.e., copper is precipitated, ferric iron is reduced, hydrogen is produced as acid is consumed.
With respect to the ferric iron reduction and hydrogen production, the consumption of iron is from 2 to 3 pounds per pound of copper being recovered from a leach solution containing about 0.6 gpl copper. However, the stoichiometric requirement is only 0.88 pounds of iron consumed per pound of copper recovered. As is evident, the cost of iron and its utilization play a significant part in the recovery of copper. Thus, after separation of the precipitated copper in the cementation process, the barren leach solution is returned to the waste dumps. This leach solution contains iron consumed in the precipitation step. In some operations, an attempt is made to precipitate the iron salts from the solution prior to the return to the leaching operation (depending on the mineralogy of the dump or the in-situ well). However, the problem associated with cementation processes has been the presence of ferric iron, which is detrimental in the cementation step because it consumes iron, that is, the ferric ion is being converted to the ferrous ion species by the iron present in the cementation process.
Inasmuch as a barren, i.e., decopperized solution from the cementation process is returned to the dump and it contains ferrous ions, rather than ferric ions, the dissolving of the copper is reduced thereby. Still further, iron which is being consumed in the cementation step is ultimately deposited in the waste dump.
A general process for leaching of copper ores is described in U.S. Pat. No. 2,563,623 which illustrates a copper recovery process as well as cementation of a pregnant liquor. U.S. Pat. No. 3,103,474 to Juda describes the recovery of metal values by electrowinning such as by hydrogen assisted (at anode) cathodic deposition of copper. In U.S. Pat. No. 3,650,925, the electrowinning of copper at porous cathodes is described; it, however, is directed to extremely dilute solutions, and the electrolyte is augmented. Moreover, the operation of the cell, the disclosed materials, and the disclosed parameters would not lend to operations, such as handling of large amounts of very dilute solutions, because the electrolyte cannot be used in a two element direct ion dependent conversion of the solution, i.e., copper recovery and multipurpose lixiviant production. Other processes are disclosed in U.S. Pat. Nos. 3,716,459; 3,657,101; 3,657,100; 3,464,904; and 3,459,646. These processes are not capable of use or adoption in an overall integrated process for recovery of copper and production of lixiviant from dilute leach solutions. The coproduction of barren leach solutions is also not disclosed as part of a unitary process.
The prior art does not teach a recovered barren leach solution for utilization in a recycling fashion for recovering copper values from copper value sources, such as in mine waste dumps or from in-situ mined copper sources. Moreover, in accordance with the present invention, the preferred pregnant liquor flow through the electrolytic cell is first through a cathode for the cathode reactions to take place and then through a porous anode or by an impermeable anode for the anode reactions to take place so that both functions of pregnant liquor depletion and lixiviant production can be properly achieved. In another embodiment the pregnant liquor may first flow through a porous anode or by an impermeable anode and then through the porous cathode to deposit the metal values in and on the porous cathode.